Is it possible to use multiple octocoupler transistors as switches connected parallel to the coil of the relay? I am worried about slight voltage differences across collector-emitter. Just like the image only multiple parallel octocouplers.enter image description here edit: I will just use transistor as somebody said in the well known OR configuration

  • \$\begingroup\$ What is the actual problem you're trying to solve? More current? Multiple switches combined with OR? \$\endgroup\$ – CL. Nov 27 '16 at 13:52
  • \$\begingroup\$ Yes i want an OR like condition \$\endgroup\$ – Piet Jan Nov 27 '16 at 14:04
  • \$\begingroup\$ With good Comm mode rejection and minimize loop area, you can have a whole building of sources pull a relay current switch or have bad wiring practice and fail in a small room. TO make ANY logic work, you must define distance source/ load impedance , SNR and error rate from interference and level of interference. E.g. a 2way light switch is XOR . but you could make it OR \$\endgroup\$ – Sunnyskyguy EE75 Nov 27 '16 at 14:20
  • \$\begingroup\$ If your problem is solved, please either accept one of the answers or write your own answer explaining the solution for future readers. \$\endgroup\$ – The Photon Nov 27 '16 at 17:07

Opto-coupler outputs in parallel is a bad idea unless you include some means for them to share current properly. Just connecting them in parallel doesn't do that.

In this case, the reason seems to be to get higher current capability on the isolated side. That can be achieved with a transistor amplifying the current thru the opto instead of using more optos.

You also seem to be using the same ground on either side of the opto. In that case, lose the opto altogether and replace it with a transistor. Those have much higher gain than optos, so a single transistor can be driven directly from a microcontroller output on one side, and drive the relay directly on the other side. Remember that relays provide isolation too. This circuit would still be isolated between the microcontroller and the switched contacts.

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    \$\begingroup\$ Im sorry for the misunderstanding but I want to use multiple analog inputs in a OR like case to power the coil. The current division is not the problem \$\endgroup\$ – Piet Jan Nov 27 '16 at 14:01
  • \$\begingroup\$ @Piet: You can run the analog inputs into the micro, then perform whatever logic you want there, then have it control the single output that drives the relay. Again, you haven't given any reason you need to isolate the relay coil in the first place. \$\endgroup\$ – Olin Lathrop Nov 27 '16 at 14:23
  • \$\begingroup\$ @OlinLathrop Perhaps OP is trying to have a number of unrelated uC control a single relay. Software would obviously be the easiest way to OR a number of signals. \$\endgroup\$ – KalleMP Nov 27 '16 at 14:48
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    \$\begingroup\$ @Kalle: Perhaps. This question is a moving target. Maybe the OP doesn't really know what he wants. \$\endgroup\$ – Olin Lathrop Nov 27 '16 at 14:53

You can parallel the outputs of several opto-couplers to do what you want but beware that output leakage current will build up and, beyond a certain point, the relay may become activated with no active signal going into one of the opto-couplers.


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